If you're ever tempted to buy cheap booze from the back of a truck, take care. It could be made from industrial alcohol that was never intended for human consumption and, as such, it might contain dangerous impurities like methanol. It is also likely to contain traces of a substance called Bitrex which is added to prevent the alcohol being drunk. Bitrex is one of the bitterest substances known, so it is added to many formulations to prevent them being consumed accidentally or on purpose. Cosmetics, household and garden products have all been protected this way, as well as ethanol that was not meant for drinking.

This type of illicit alcohol can be identified from the presence of Bitrex, which is denatonium benzoate, but this can be destroyed by adding sodium hypochlorite, so often only traces remain. When this happens, the low concentrations can be detected by Raman spectroscopy, say Polish researchers writing in Journal of Forensic Sciences. Not regular Raman, in which the bands originating from ethanol mask the spectrum, but surface-enhanced Raman spectroscopy (SERS) which is more sensitive and can see the small levels of Bitrex.

With the support of a supervised learning model to process the data, the spectra of normal vodka could be easily differentiated from those of vodka contaminated with Bitrex and treated with hypochlorite. The successful detection rate was 98% and the results were available within a few minutes with no sample preparation.

The research team recommend the use of SERS for cunsumer protection as an alternative to GC/MS or IR spectroscopy due to the ease of analysis and the ability to use a portable spectrometer.